Awesome, I will wait to see what you do for the adaptation of the sten mag. I have a lower that I was thinking about making a .45 ACP upper for but my kid says I may have trouble with finding a reliable mag to use that will fit the ar lower. I do reload for the 9mm as well as the .45 so maybe I'll follow your lead with 9mm first. BTW thanks for posting your adventure!

Do it before it is too late!
Proud member of the NRA,GOA and JPFO!!!
And now GOCRA in MN!!!!

Wrench, Hahn makes an adaptor to use sten mags, and they also sell one for grease gun mags. Don't know if the grease gun adaptor is a drop in or not. I'm leaning towards making an adaptor like the hahn. There are two styles of these. One thats basically a block like the Olympic mag adaptors, and another one that is ar15 mag profile and the mag sets about in the middle of the mag well. I'm going to make the block type.

I got some more work done on the 9mm build. I have been able to get it to fire semi-auto using the sten mag with a block of plastic jammed behind the mag to hold it in place temporarily! Since it did run, I went ahead and profiled the gas block so that it would fit under a handguard. The front sight is pressed on and held with a set screw at this point. Once I'm ready to parkerize it I'll drill the front sight pin holes. When I get time after my mag block is done I need to make a flash hider for it, but in the mean time the one thats on there will protect the barrel threads. Here's some pics.

Really nice! Are you still at .090 for the gas port? The profile job on the block looks great.
The sten mag looks great also! Show pics of your mag block when its done. This is a really cool build and I really like it, good job so far!

Do it before it is too late!
Proud member of the NRA,GOA and JPFO!!!
And now GOCRA in MN!!!!

Wrench, I actually opened up the gas port some, but it had no effect. So I removed the buffer weights and that got it working. I think since the gas tube is a little under 1/8" on the inside, only so much gas can get down the tube no matter how big the port is. I tried to contour the gas block much like it was layed out on the first blocky picture of it. I used a regular end mill to remove the corners, and then a 3/8" ball end mill to contour the top. I'm sure I have a couple 1/2" double ended carbide ball end mills somewhere, but haven't seen them since the move. I'm sure the shop gnomes will make them appear in my sock drawer or china cabinet in a month or two.

I forgot to mention earlier that there are two styles of original sten mags. One will accept a 45acp with a little tweaking, and the other will not. The sten mags I have will not accept a 45 round so I can't speak of what the second style looks like. I've seen a thread on the subject recently either here or on weaponsguild. The black plastic block will work as is if screwed to the back of one of the sten mags. I think I'm going to mill a slot in one for a mag catch though.

j427x, I haven't made a magwell block yet, but just by wedging a piece of plastic behind the mag to temp hold it in place I've been able to load up several rounds in a sten mag and shoot them. I haven't loaded up a full mag yet as I figured the mag would move before I got to the end of it anyway. I cast lead rn bullets in 9mm and I'm uncertain if I'll be able to use them as when I pull the bolt back slowly the bolt lugs dig into the lead bullets of my dummy rounds. It doesn't seem to effect the hp bullets. Yes it feeds the winchester ranger hollow points I've been testing with! I'll have to load some lead bullets up to see if they feed ok and how they work out with the gas port and if they shave off lead down into the gas system or not. I might be able to limit the effect of the lead bullet deforming by lower the mag as well. Heck it might not even do it when it shoots. Only one way to find out.

I ordered some 1"x1.25" aluminum bar to make a magwell block out of, but it'll be at least a week till it gets here. So in the mean time I rummaged around and found some 1/2" aluminum plate and cut out a couple 1.3" strips with my trusty hacksaw. After cleaning up my hacksaw job on the mill and screwing two pieces together. Squaring the darn thing up and screwing it together took up alot of time. When the aluminum bar gets here I'll make another one since it won't take quite as long.
I was able to fit the block to a 80% tactical lower and was happy until I tried to fit it in another lower without success, then another, then another, ect... Back to the mill to remove a bit more so the block would fit in all of my lowers. I still need to slot it for both mag catches, and trim what sticks out of the bottem. Then of course make a mag catch to hold the sten mags.

I managed to finish a magwell block. It was a comedy of errors none of which were critical. The first was measuring the sten mag catch pocket location and then milling on the wrong side of the line I also lined up the mag catch with a chip and not a layout line But no deal breaker mistakes and I ended up with a functional magwell block.
I test fired a bunch of rounds and was getting on average about 2-3 rounds before the gun jammed. I have been using an old box of winchester ranger 147gr JHP, like the black talons only not black. I switched over to fmj and managed to get about 5 rounds off before the gun jammed again. A piece of copper jacket managed to find its way into the carrier and made the bolt turn hard. Thats not unusual for metal shavings to get in the carrier for the first few rounds so I'm not worried. I've had it happen before on 556 uppers and it usually damages the gas rings. These rings still look good though. I'll give it another shot tomorrow if I can get away again.

I got to shoot the upper today. I took two 30rnd mags to the woods with me. The first mag I had a failure to feed at round #29. I loaded it back up and shot the last two rounds without any problems. Then I tried the second sten mag I have. It's a well used mag with what looks like at least 3 layers of paint on it. After about 6-7 failures to feed where the bullet tip either tipped down and jammed on the front of the mag or tipped down, but not far enough to jam, but the back of the casewould be lower than normal since the rounds under it weren't pushing it up, causing it to tip upwards and jam on the top of the barrel inside the barrel extension. I took about 15 rounds out and tried again, but I could not get that mag to feed at all. So I put all of the loose rounds into the first "good" mag and they all shot off without incident except for two which had been damaged from the first load attempt with the bad mag.
A little bit more work to do to get it feeding 100% but it's almost there.